Horizontal Method (With and without lights on) Pictures 3 & 4
Vertical Method (With and without lights on )Pictures 5 & 6
Hopefully you can see the difference in the hanging. The horizontal lights create an obvious swirl effect (as seen in the pictures) leaving gaps of tree in the dark, and would probably be difficult to fill in with extra lights. The vertical light method, which seemed more time intensive, appears to be the give most comprehensive coverage. (Note that towards the bottom right is where the strand ran out of lights!) Additionally, it was easier than walking around and around the tree hoping you got in the right set of branches.
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More commentary and side thoughts on original post: http://sparklysharpandfabulous.blogspot.com/2011/12/art-of-lighting-christmas-tree-vertical.html